1. Field of the Invention
The present invention relates to a substrate for circuits, such as one for a flat antenna to receive satellite-broadcast microwaves, which is fitted for use in a high frequency region. The invention also relates to a process for making such substrates.
2. Description of the Prior Art
As described in Nikkei Electronics, No. 347, pp. 145-160 (1984, issued by Nikkei-MeGraw Hill Corp.), flat antennas for receiving satellite-broadcast microwaves or other microwaves include those of a type in which wave power is received by a number of resonators arranged on copper foils or the like laid on one side of a dielectric substrate and the received power is matched through micro-strip lines, collected at one or more sites, and led through coaxial cables to a converter, tuner, or the like. However, the high-frequency power, on passage through a conductor including dielectric substances, is much attenuated by the surface resistance and the like, that is, the wave power from the resonators is lost greatly when propagated through the strip lines (the transmission loss is large). A conceivable method for reducing this transmission loss comprises dividing the opening of the flat antenna into several sections, reducing the lengths of the strip lines, collecting the wave power from the resonators at the centers of these sections, and connecting these sections together through coaxial cables of small transmission loss. This method, however, has drawbacks in that the connection of strip lines with coaxial cables is complicated and a large number of works are necessary for the fabrication. Accordingly it is desirable that a number of resonators be arranged on a substrate, connected together at one site through micro-strip lines, and coupled with a coaxial cable. For this purpose the substrate should be provided with micro-strip lines through which the transmission loss is small. The transmission loss is the ratio, expressed in dB, of the transmitted power to the supplied power per 1 m of the transmission path, that is, the loss is represented by the formula ##EQU1## wherein X is the transmitted power when the supplied power is taken as 100. For instance, a transmission loss of 2, 3, or 4 dB/m means that 63, 50, or 40% of the power supplied at one end of a transmission path is transmitted by 1 m of the path and 37, 50, or 60% of the power is lost during the passage therethrough. Waves used in satellite brood casting are faint since such high power cannot be supplied in this case as in the case of ground broadcasting where VHF or UHF is used, so that substrates of small transmission loss are required in order to increase antenna gains as well. In order to reduce the transmission loss, it is necessary to use a substrate having a low dielectric constant and a low dielectric loss tangent.
Therefore the insulating layers of such substrates are formed of materials having low dielectric constants and dielectric loss tangents, such as polytetrafluoroethylene, polyethylene, polypropylene, polystyrene, polyisobutylene, and polymethylpentene-1 and of minimized amounts or none of such reinforcements as glass fiber, paper, and the like since these reinforcements have high dielectric constants and dielectric loss tangents.
On the other hand, in order to obtain circuits having a low dielectric constant and a low dielectric loss tangent there are proposed the method of incorporating hollow micro-spheres (micro-baloons) (Japanese patent application Laid-Open No. 167394/85) and the method of blending quartz glass fiber with glass fiber that is a base material which is impregnated with a synthetic resin and heated and pressed to produce laminates (Japanese patent application Laid-Open No. 109347/84).
Recently, frequencies of waves for use in the fields of electronic industry and communication industry have shifted gradually to higher-frequency regions and interest has moved from the region of kilohertz that have hitherto been used extensively to the region of megahertz or gigahertz. In these higher-frequency regions, energy loss in wave transmission is large and hence materials of lower dielectric constants and loss tangents have been desired.
However, when polytetrafluoroethylene or polyethylene, which has a low dielectric constant and a low dielectric loss tangent, is used for the substrates, the process for fabricating antennas will be complicated and additionally there is a limit of decreasing the transmission loss, so that none of satisfactory substrates have been obtained.
As regards the method of incorporating hollow micro-spheres, a number of operation steps are required and if the dispersion of the hollow micro-spheres in the insulating layer is insufficient, dielectric properties of the resulting substrate for circuits will be locally uneven. In addition, when the adhesion of the hollow micro-spheres to the matrix of the insulating layer is insufficient, water will penetrate into the layer and increase the dielectric constant and the dielectric loss tangent. Moreover, the dielectric constant and the dielectric loss tangent are not so much decreased with an increase in the content of hollow micro-spheres, since materials of relatively high dielectric constants and dielectric loss tangents are used in most cases for the micro-spheres. Laminates comprising quartz glass fiber as a base material are expensive and in addition it is limited to decrease the dielectric constant and the dielectric loss tangent by using such laminates.
Since the thermal expansion or contraction of gases is larger than that of solids, circuit-supporting substrates containing hollow micro-spheres, which are closed cells, undergo large dimensional changes in the directions of length, width, and thickness as the gas in the spheres expands or contract with seasonal and daily variations in temperature. Changes in the warp and thickness of these substrates are larger than in those of circuit-supporting noncellular substrates.
For instance, when a circuit-supporting substrate containing hollow micro-spheres in the insulating layer is used for a flat antenna to receive microwaves, said warp causes problems such as a shift of received wave phase. A change in the thickness also raises a problem in that the range of receivable wave frequencies is shifted thereby.